Jetpack Photon should help
I recommend Jetpack-Photon module (deactivate all the other Jetpack modules you don’t use). Photon is an image-only CDN solution using Automatic’s servers. All your images are uploaded to their servers the next time the image is served after you activate Photon.
Subsequently the images are served from the CDN network concurrently with your content being served from your host. Photon uploads the original size image, but serves a scaled image based on the browser request. It’s smart enough to see the difference between a Retina and lower-res display device and serves a higher res image to the Retina display. You can verify this by opening the image in another tab and looking at the url. If Photon is serving it the url will start with something like i2.wp.com/yourdomain… Try it out by right-clicking on the image in this post for example.
This one trick without any other caching makes a HUGE page load difference on sites with lots of graphics, especially if the images have not been pre-optimized. This is not an excuse for you to avoid pre-sizing and optimizing your images before you upload them to your site. But if you have a lot of legacy images that are way too big, Photon can help.
Two caveats, 1) once the image has been uploaded there is no way to purge the CDN cache, so if you replace the image with another, but use the same file name, the original image will still get served (so always change the file name if you are replacing an image). 2) the WordPress “feature” where a featured image on a page or post will replace the default header if the featured image is as wide or wider than the header image will not work if you use Photon. This is a known bug, but has not been fixed.
Give it a try. It’s free and only takes a minute to turn Photon on and try it out. Try it on a page with a lot of images. Sometimes you may have to reload the page several times until all the images are cached by Photon. If you don’t like it, just deactivate the Photon module or the Jetpack plugin. No harm done.
By the way, if you like the idea of using Photon, but don’t want to use Jetpack, and are willing to hack your functions.php file, check out this post on the WP Actions website: How to use Jetpack Photon Without Jetpack Installed.